In recent years, the growth of the nursery and lawn care industries has created a demand for compact, inexpensive, yet accurate fluid proportioning and mixing systems. Ideally, a system of this type should operate effectively to mix, in a preset proportion, a chemical with a stream of solvent, such as water, and the ratio of chemical to water must be accurately maintained for a normal range of variables which effect both the water and chemical supplies. The system should accommodate both a wide range of primary flows and also provide the ability to adjust readily and continuously the proportioning ratio over a range of continuously desirable settings. In addition, the proportioning and mixing unit must be compact and formed of material which is both inert to a wide range of chemical ingredients normally mixed with water and must remain operative over a wide range of temperatures. The device should be driven by power derived from the water supply without creating a substantial pressure drop in the water supplied as a solvent, and the water supply should be positively protected from backflow contamination by the chemical used as a solute.
Such a fluid proportioning and mixing system is shown by our previous U.S. Pat. No. 4,832,072. This patented fluid proportioning and mixing unit includes a metering and pumping section in combination with a mixer unit, and the mixer unit operates effectively with an input water flow of 50 gallons per minute. A similar mixer unit might operate effectively at a higher capacity unit if effective filtering could be achieved. However, commercially available filters which will satisfactorily filter the entire input base fluid flow to a high capacity mixer unit of the type shown in our prior patent are extremely expensive and bulky and would present routine maintenance problems. Consequently, it has not been practical to provide a large capacity mixer unit of the type shown in our previous patent due to the expense and mechanical disadvantages involved in providing a filter to filter the entire large capacity flow through the mixer.